Hydrogenation is a well-established process both in the chemical and petroleum refining industries. Hydrogenation is conventionally carried out in the presence of a catalyst which usually comprises a metal hydrogenation component on a porous support material, such as a natural clay or a synthetic oxide. Nickel is often used as a hydrogenation component, as are noble metals such as platinum, palladium, rhodium and iridium. Typical support materials include kieselguhr, alumina, silica and silica-alumina. Depending upon the ease with which the feed may be hydrogenated, the hydrogen pressures used may vary from quite low to very high values, typically from about 100 to 2,500 psig.
Hydrogenation is an exothermic process and is therefore thermodynamically favored by lower temperatures but for kinetic reasons, moderately elevated temperatures are normally used and for petroleum refining processes, temperatures in the range of 100.degree. to 700.degree. F. are typical. Hydrogenative treatment is frequently used in petroleum refining to improve the qualities of lubricating oils, both of natural and synthetic origin. Hydrogenation, or hydrotreating as it is frequently termed, is used to reduce residual unsaturation in the lubricating oil, and to remove heteroatom-containing impurities and color bodies. The removal of impurities and color bodies is of particular significance for mineral oils which have been subjected to hydrocracking or catalytic dewaxing. For both hydroprocessed mineral and synthetic stocks, the saturation of lube boiling range olefins is a major objective.
The polyolefins comprise one class of synthetic hydrocarbon lubricants which has achieved importance in the lubricating oil market. These materials are typically produced by the polymerization (the term oligomerization is often used for the lower molecular weight products which are used as low viscosity basestocks) of alpha olefins typically ranging from 1-octene to 1-dodecene, with 1-decene being a preferred material, although polymers of lower olefins such as ethylene and propylene may also be used, including copolymers of ethylene with higher olefins, as described in U.S. Pat. No. 4,956,122 and the patents referred to there. The poly alpha-olefin (PAO) products may be obtained with a wide range of viscosities varying from highly mobile fluids of about 2 cS at 100.degree. C. to higher molecular weight, viscous materials which have viscositics exceeding 100 cSt at 100.degree. C. The PAO's are conventionally produced by the polymerization of the olefin feed in the presence of a catalyst such as aluminum trichloride, or boron trifluoride or trifluoride complexes. Processes for the production of PAO lubricants in this way are disclosed, for example, in U.S. Pat. Nos. 3,382,291 (Brennan), 4,172,855 (Shubkin), 3,780,128 (Shubkin), 3,149,178 (Hamilton), 3,742,082 (Brennan), and 4,956,122 (Watts). The PAO lubricants are also discussed in Lubrication Fundamentals, J. G. Wills, Marcel Dekker Inc., New York, 1980 ISBN 0-8247-6976-7, especially pages 77 to 81. Subsequent to the polymerization, the lubricant range products are hydrogenated in order to reduce the residual unsaturation. In the course of this reaction, the bromine number of the lubricant is reduced from typical values of about 30 or higher for low viscosity PAOs and 5 to 15 for high viscosity PAOs to a value of not more than about 2 or even lower.
A novel type of PAO lubricant has recently been disclosed as having exceptional and advantageous properties. These materials are the HVI-PAO materials which are disclosed in U.S. Pat. Nos. 4,827,064 (Wu), 4,827,073 (Wu). These materials are produced by the oligomerization of alpha olefins, especially 1-decene, using a reduced Group VI metal oxide catalyst, preferably a reduced chromium oxide catalyst. The HVI-PAO products may be derivatized by reaction with aromatics, as disclosed EP 377305 and higher molecular weight versions prepared by the use of lower oligomerization temperatures, as disclosed in U.S. Pat. No. 5,012,020, to which reference is made for a disclosure of the higher molecular weight products. Although the oligomerization process using the reduced metal oxide catalysts produces a material of characteristic structure, residual unsaturation remains in the oligomer product and, like the conventional PAO oligomers, it is subjected to hydrogenation in order to improve its stability as a lubricant. The hydrogenation is carried out in the same manner as with the conventional PAO-type materials.
The catalysts used for hydrogenating lubricants, whether of mineral oil or synthetic origin, require a strong hydrogenation function provided by the metal component and an effective large pore diameter in the porous support material in order to minimize the diffusion resistance of the bulky lubricant molecules. For reactions with bulky molecules, the optimum ratio of catalytic pore diameter to molecule size is about 1.5:1. Table 1 below shows the optimum pore sizes required for normal alkanes in the C-7 to C-25 range. The table shows that for alkanes in this range, the chain-length varies from 9.9 to 37.6 .ANG. so that active hydrogenation catalysts for these materials should have a major amount of their pore volume with pore openings in the range of 15 to 56 .ANG., and preferably with a major amount of this in the range 38 to 56 .ANG..
TABLE 1 ______________________________________ Optimum Pore Size Optimum Pore Carbon Number Alkane Length, .ANG..sup.(1) Diameter, .ANG..sup.(2) ______________________________________ C.sub.7 9.9 15 C.sub.8 11.5 17 C.sub.10 14.5 22 C.sub.12 17.6 26 C.sub.17 25.3 38 C.sub.19 28.4 43 C.sub.21 31.5 47 C.sub.23 34.6 52 C.sub.25 37.6 56 ______________________________________ .sup.(1) Based on bond lengths of 1.54 and 1.11 .ANG. for C--C and C--H bond lengths, respectively. .sup.(2) Based on an optimum ratio of catalyst pore to molecule size rati of 1.5.
Conventional amorphous support materials such as alumina, silica and silica-alumina, typically have a pore size distribution with most of the pores larger than 50 .ANG. and most of these are larger than 100 .ANG.. Although these large pores enable the bulky lubricant molecules to traverse the molecular structure of the catalyst freely with little diffusional resistance, the reduced surface area associated with the larger pore sizes diminishes the area which is available for the hydrogenation reactions. It would therefore be desirable to utilize a hydrogenation catalyst which possess a significant amount of its pores in the range of 15 to 60 .ANG., close to the optimum ratio for the lower molecular weight materials making up the bulk of many synthetic lubricants as well as the lower viscosity mineral oils.